Ultralow-velocity zone geometries resolved by multidimensional waveform modelling

Vanacore, Elizabeth; Rost, Sebastian; Thorne, M. S.

doi:10.1093/gji/ggw114

Cited by 16 publications

(12 citation statements)

References 57 publications

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“…To determine the most parsimonious distribution of heterogeneities, we select a grid size of either 4° or 8°. The smallest grid size of 4° is selected as a grid size in which we can solve the inverse problem for multiple solutions in a reasonable amount of time, and is also of the order of the smallest heterogeneities that previous studies have demonstrated could produce the anomalous SPdKS waveforms (Jensen et al, 2013;Thorne et al, 2013Thorne et al, , 2019Vanacore et al, 2016).…”

Section: Resultsmentioning

confidence: 99%

“…The SPdKS wavefield is complex in the presence of a ULVZ. The generation of highly anomalous waveforms appears to be due to (1) constructive interference of scattered arrivals from ULVZ edges (see, e.g., Thorne et al., 2019; Vanacore et al., 2016) and (2) the generation of additional Pd energy from converted phases such as SP ULVZ PdKS (see Thorne et al., 2020). It is unclear why more anomalous SPdKS‐like energy is observed for ULVZs located on the source‐sides of the path in the synthetic computations we have done thus far.…”

Section: Discussionmentioning

confidence: 99%

“…ULVZs were first inferred using the SPdKS seismic phase (Garnero et al., 1993). SPdKS has subsequently been used to infer the ULVZ presence in multiple studies (Jensen et al., 2013; Ni & Helmberger, 2001; Rondenay & Fischer, 2003; Thorne & Garnero, 2004; Vanacore et al., 2016; Wen & Helmberger, 1998). What is commonly referred to as SPdKS is a combination of complementary arrivals called SPdKS and SKPdS, as shown in Figure 1.…”

Section: Introductionmentioning

confidence: 99%

“…However, the existence of a lower mantle structure such as a ULVZ, on either the source‐ or receiver‐side can cause perturbations in timing for either SPdKS or SKPdS. But, in most cases it may be impossible to see separate SPdKS and SKPdS arrivals (Rondenay et al., 2010; Vanacore et al., 2016). Even if separate SPdKS and SKPdS arrivals are distinguishable, it is not readily apparent whether the structure giving rise to the anomalies occurs as (1) a source‐side ULVZ, (2) a receiver‐side ULVZ, or (3) a combination of both source‐ and receiver‐side ULVZs.…”

Section: Introductionmentioning

confidence: 99%

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The Most Parsimonious Ultralow‐Velocity Zone Distribution From Highly Anomalous SPdKS Waveforms

Thorne

Leng

Pachhai

et al. 2021

Geochem Geophys Geosyst

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For nearly 3 decades, features sitting on top of the core-mantle boundary (CMB) and referred to as ultralow-velocity zones (ULVZs) have been invoked to explain anomalous travel-time delays and the existence of a variety of pre-and post-cursor arrivals observed in seismic data. A clear definition of what constitutes a ULVZ is currently lacking; nevertheless, most papers invoking ULVZ occurrence state that they are regions of low S-and/or P-wave velocities (of the order of 10% or greater decreases) in thin regions (typically less than 40 km is stated) sitting on top of the CMB (

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Section: Resultsmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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The Most Parsimonious Ultralow‐Velocity Zone Distribution From Highly Anomalous SPdKS Waveforms

Thorne

Leng

Pachhai

et al. 2021

Geochem Geophys Geosyst

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“…Hence, it appears unlikely that the anomalous waveforms we observe result from slab structure. (3) We only consider box-car-shaped ULVZs in 2-D. Variations in 2-D ULVZ shape alter waveform characteristics (Vanacore et al, 2016), and it is expected that 3-D ULVZ shapes will as well. Computing high-frequency synthetic seismograms for 3-D models is currently possible, albeit at high computational cost (Leng et al, 2016), and will be important in future efforts.…”

Section: Discussionmentioning

confidence: 99%

Melting at the Edge of a Slab in the Deepest Mantle

Thorne

Takeuchi

Shiomi

2019

Geophysical Research Letters

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We analyzed new recordings of SPdKS seismic waveforms from a global set of broadband seismograms and horizontal tiltmeters from the Hi‐net array in Japan from 26 earthquakes in the Central American region. The anomalous waveforms are consistent with the presence of at least three ultralow‐velocity zones (ULVZs), on the core‐mantle boundary beneath northern Mexico and the southeastern United States. These ULVZs ring an area of high seismic wave speeds observed in tomographic models that has long been associated with past subduction. Waveform modeling using the PSVaxi method suggests that the ULVZs have S and P wave velocity decreases of 40% and 10%, respectively. These velocity decreases are likely best explained by a partially molten origin where the melt is generated through melting of mid‐ocean ridge basalt atop the subducted slab.

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Ultralow Velocity Zone Locations: A Global Assessment

Garnero

2018

Geochem Geophys Geosyst

129

120

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We have compiled all previous ultralow velocity zone (ULVZ) studies, and digitized their core‐mantle boundary (CMB) sampling locations. For studies that presented sampling locations based on infinite frequency ray theory, we approximated Fresnel zones onto a 0.5° × 0.5° grid. Results for these studies were separated according to wave type: (1) core‐reflected phases, which have a single location of ULVZ sampling (ScS, ScP, PcP), (2) core waves that can sample ULVZs at the core entrance and exit locations of the wave (e.g., SPdKS, PKKP, and PKP), and (3) waves which have uncertainties of ULVZ location due to long CMB sampling paths, e.g., diffracted energy sampling over a broad region (Pdiff, Sdiff). For studies that presented specific modeled ULVZ geographical shapes or PKP scatter probability maps, we digitized the regions. We present summary maps of the ULVZ coverage, as well as published locations arguing against ULVZ presence. A key finding is that there is not a simple mapping between lowermost mantle reduced tomographic velocities and observed ULVZ locations, especially given the presence of ULVZs outside of lowermost mantle large low velocity provinces (LLVPs). Significant location uncertainty exists for some of the ULVZ imaging wave types. Nonetheless, this compilation supports a compositionally distinct origin for at least some ULVZs. ULVZs are more likely to be found near LLVP boundaries, however, their relationship to overlying surface locations of hot spots are less obvious. The new digital ULVZ database is freely available for download.

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Ultralow-velocity zone geometries resolved by multidimensional waveform modelling

Cited by 16 publications

References 57 publications

The Most Parsimonious Ultralow‐Velocity Zone Distribution From Highly Anomalous SPdKS Waveforms

The Most Parsimonious Ultralow‐Velocity Zone Distribution From Highly Anomalous SPdKS Waveforms

Melting at the Edge of a Slab in the Deepest Mantle

Ultralow Velocity Zone Locations: A Global Assessment

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